The application relates generally to screw compressors. The application relates more specifically to screw compressors capable of operating at increased pressures.
Heating and cooling systems typically maintain temperature control in a structure by circulating a fluid within coiled tubes such that passing another fluid over the tubes effects a transfer of thermal energy between the two fluids. A primary component in such a system is a compressor which receives a cool, low pressure gas and by virtue of a compression device, exhausts a hot, high pressure gas. One type of compressor is a screw compressor, which generally includes two cylindrical rotors mounted on separate shafts inside a hollow, double-barreled casing. The side-walls of the compressor casing typically form two parallel, overlapping cylinders which house the rotors side-by-side, with their shafts parallel to the ground. Screw compressor rotors typically have helically extending lobes and grooves on their outer surfaces forming a large thread on the circumference of the rotor. During operation, the threads of the rotors mesh together, with the lobes on one rotor meshing with the corresponding grooves on the other rotor to form a series of gaps between the rotors. These gaps form a continuous compression chamber that communicates with the compressor inlet opening, or “port,” at one end of the casing and continuously reduces in volume as the rotors turn and compress the gas toward a discharge port at the opposite end of the casing for use in the system.
During operation, due to the difference in pressures 80, 82 between the respective inlet and outlet openings or ports, also referred to as inlet 81 and outlet 83, the resulting generated forces 86 are reacted by bearings secured in the housing (FIGS. 1 and 2) near opposed ends 90, 91 of the rotors 92, 94. One way to further increase operating pressures and differences between the inlet and outlet pressures 80, 82 is to apply larger bearings or add more bearings in parallel. However, there are significant challenges associated with increasing the forces generated by the rotors during their operation. As shown in FIG. 2, the size of the bearings, i.e., the diameter (“DM”) of the bearings 88 associated with the male rotor 92 and the diameter (“DF”) of the bearings 88 associated with the female rotor 94 is related to the distance (“CD”) between the rotational axes 96, 98 of the respective male rotor 92 and the female rotor 94 as identified in equation 1:(DM+DF)/2<CD  [1]
In other words, one half of the sum of the diameter DM of the bearings 88 associated with the male rotor 92 and the diameter DF of the bearings associated with the female rotor 94 must be less than the distance CD between the rotational axes 96, 98 of the male rotor 92 and the female rotor 94. Unfortunately, bearing load carrying capability is related to its diameter, and current designs are approaching the upper limits of bearing load carrying capability for the largest bearing sizes that may be used.
In addition, the solution cannot be achieved by adding bearings in a side-by-side 104 arrangement to each end of the rotors, for several reasons. First, as shown collectively in FIGS. 3-4, even bearings 88 having identical part numbers can have different clearances 100, as well as different interferences 102 with the rotor shaft 106. As a result, it is extremely difficult for bearings 88 positioned side-by-side 104 to each other to be parallel to each other and share in supporting the operating loads. Second, even if the bearings 88 positioned side-by-side 104 to each other are parallel, due to the deformation of the rotor 92, 94 (rotor 94 shown in FIG. 5) under load (FIG. 5 is not to scale to assist in understanding the effect of rotor deformation), the ends 90, 91 of the rotors 92, 94 would still not be parallel to the respective axis of rotation of each rotor. Therefore, under such operating conditions, it is impossible for conventional bearings 88 positioned side-by-side 104 to reliably and/or meaningfully share in supporting the operating loads. Worse yet, if the rotor 92, 94 deflection is sufficiently large, shear loads 108, due to misalignment are created for which the bearings 88 are not designed to withstand, resulting in premature failure of the bearings 88, and at the least, down time of the screw compressor, if not risk of damage of other screw compressor components.
Accordingly, there is an unmet need for reliably and inexpensively supporting increased operating loads of screw compressors.